In order to use aluminum sheets as supports for lithographic printing plates, the surfaces of the aluminum sheets are usually roughened for improving the adhesion with photosensitive layers formed thereon and also improving the water retensivity thereof. The roughening treatment is usually called "graining" and this treatment is inevitably required in the production of supports for lithographic printing plates; furthermore, considerable operation skill has been required for performing this treatment.
Graining for aluminum sheet is generally classified into mechanical graining such as ball graining, wire graining, brush graining, etc., and electrolytic graining, usually referred to as electrochemical etching. Ball graining requires skill factors for selecting the materials of ball, the kind of abrasives, the control of the amount of water during polishing, etc.; also, it is difficult to perform continuous ball graining treatment, or, in other words, each sheet must be finished by an individual, discontinuous, treatment. The employment of wire graining results in non-uniform grains. On the other hand, brush graining can overcome these difficulties, but has such faults that the grains formed are generally simple and shallow, rotary brush patterns remain on the surface of aluminum sheets thus brush grained, sometimes some directional property appears for grains formed, and non-image areas are apt to be stained.
An electrolytic graining treatment can provide uniform grains having large mean roughness as compared to conventional mechanical graining methods such as ball graining, brush graining, etc., by properly selecting the electrolytic conditions, but the conditions are restricted very severely. In more detail, if various conditions such as the composition and temperature of the electrolytes, the electrolytic conditions, wetc., are kept constant, products having constant qualities are easily obtained, but these electrolytic conditions are very severe and it is very difficult to control these conditions within proper ranges. Furthermore, when the surfaces of aluminum sheets are grained by an electrolytic graining treatment, there is an economical problem in that a large amount of electric power is consumed, and hence the cost for the electric power in the production costs for lithographic printing plates becomes very high. Moreover, in the electrolytic graining treatment, the waste electrolyte used in the electrolytic treatment contains a considerable amount of aluminum ions, and hence the personal expense as well as the cost for chemicals required for the treatment of the waste solution becomes very high.
As a means for somewhat overcoming these difficulties, a method of producing supports for lithographic printing plates is disclosed in Japanese Patent Publication (OPI) No. 123204/78 in which an aluminum sheet grained by brush graining is further grained by alternating current electrolytic graining at a quantity of electricity of 2,000 coulombs or less in an acid electrolyte, thereby obtaining a superimposed grained surface by the combination of brush graining and electrolytic graining applied to the surface of the aluminum sheet.
However, it has been found that in such a superimposed graining method including brush graining and electrolytic graining, when the electrolytic graining step practically disclosed in the specification of the abovementioned patent publication is practiced that is, when the electrolytic graining treatment is practiced in an electrolyte containing hydrochloric acid as the main component using a sinusoidal symmetrical alternating current whose potential at anodic period is equal to that at cathodic period and whose quantity of electricity during the anodic period is equal to that during the cathodic period, the saving on consumed electric power may be achieved as compared with the case of graining the surface of an aluminum sheet by an electrolytic graining treatment only, but practically the properties of the aluminum sheet as supports for lithographic printing plates are not improved.